As a method for optically determining a three-dimensional shape of an object (target object), in the related art, various methods have been developed and used, including a stereoscopic imaging method, which is a passive shape measurement method; and a light-sectioning method and a spatial coding method, which are active shape measurement methods. However, in all of these methods, the shape cannot be measured for a portion of the object which is at a backside in relation to the camera or for a portion of the object which is in a blind spot.
In consideration of this, in some cases, for example, multiple cameras are provided on a frame surrounding the object, images from multiple viewpoints are obtained by the cameras, and information of the images are integrated through, for example, a stereoscopic imaging method to calculate the three-dimensional shape. However, this method has a disadvantage in that the equipment becomes large-scaled.
In a device described in Patent Document 1, an object is placed on a turntable, and images of the object on the turntable are captured by a three-dimensional camera while the turntable is rotated. With this process, a three-dimensional shape (depth image) viewed from a plurality of viewpoints is obtained by means of a single camera, and, by integrating the images, a three-dimensional shape is calculated with a smaller number of blind spots. This method also requires a relatively large device; i.e., the turntable.
In another known method, images of an object are captured from various positions while a camera is moved, and a three-dimensional shape of the object is calculated from the thus-obtained images from the multiple viewpoints. In this method, in place of using multiple cameras, one camera is used while being moved. However, the information from the multiple viewpoints cannot be integrated unless the positions and orientations of the moved camera are known. Although the position and orientation of the camera at each image-capturing point can be easily determined by employing a mechanism to move the camera along a track, such a mechanism would increase the size of the device.
In contrast to the related art techniques which require relatively large-scale devices, in recent years, research has been conducted on methods in which the three-dimensional shape of an object is recovered from images captured from free positions by holding a camera in, for example, a hand. In this case, the position and orientation of the camera at each image-capturing position must be determined. One of the methods for determining the position and orientation is a method known as camera calibration.
Various camera calibration methods are known. A representative calibration method is, for example, shown in Patent Document 2, which uses a calibration panel on which a plurality of patterns which differ in shape from each other (such as numbers, barcodes, etc.) are provided. In this related-art technique, an image of the calibration panel and the object are simultaneously captured in the field of view of the camera, calibration software identifies the patterns within the captured image by reference to known information regarding the shapes of the patterns on the panel and positions of the patterns in a global coordinate system, and coordinate transformation parameters between a screen coordinate system and the global coordinate system are calculated on the basis of a correspondence between the positions of the patterns on the two coordinate systems. The coordinate transformation parameter thus represents the position and the orientation of the camera with respect to the panel.
As another method, Patent Document 3 discloses a method in which a plurality of markers made of LEDs (Light-Emitting Diodes) are provided on an upper surface of a measurement head which measures a three-dimensional shape through the light-sectioning method, and an image of a measurement head which is freely placed with respect to the object is captured from above by means of a stereoscopic camera, thereby calculating the position and the orientation of the measurement head in the global coordinate system. In this method, coordinates of various points on the object in a coordinate system centered on the measurement head and measured by the measurement head are transformed by reference to information of the position and orientation of the measurement head determined by the stereoscopic camera, to thereby determine the shape of the object in the global coordinate system.
In the method of Patent Document 2, the shape of the markers (patterns) drawn on the calibration panel would change in response to a relative tilt of the panel with respect to the camera. When the shape of the marker changes, the shape would differ from the shape of the marker registered in the calibration device, and, thus, the identification precision of the markers on the panel is lowered, resulting, in some cases, in inability to calibrate. In addition, although another method is proposed in the related art in which the markers are determined not by shape but by color, the color of the markers on the panel would change due to conditions of the illumination light or the like, and, thus, there has been a problem in that the identification precision of the markers may be lowered depending on the illumination conditions.
Meanwhile, in the method of Patent Document 3, because an LED which emits light is used as the marker for determining the position and orientation of the measurement head, the influences of the difference in illumination on the accuracy of marker identification can be significantly reduced. However, in this method, a single LED is used for a plurality of markers, and the individual marker is identified not on the basis of a characteristic of each marker, but rather on the basis of the placement pattern of the markers on the upper surface of the head. Because of this, when any of the markers on the upper surface of the head cannot be imaged by the stereoscopic camera because of, for example, a shadow of the object moving over the marker, it becomes substantially impossible to automatically determine to which of the plurality of markers the marker being imaged corresponds.    [Patent Document 1] Japanese Patent Laid-Open Publication No. 2002-098521    [Patent Document 2] Japanese Patent Laid-Open Publication No. 2001-264037    [Patent Document 3] Japanese Patent Laid-Open Publication No. 2001-241927